Abstract

Ailanthus altissima (Mill.) Swingle and Robinia pseudoacacia L. are two aggressive invasive trees in riparian areas in Central Spain. We aim to test whether soil properties, nutrient mineralization rates and soil bacterial communities of riparian forest dominated by the native Populus alba L. can be altered by the presence of A. altissima or R. pseudoacacia. In autumn 2011 and spring 2012 we conducted a field soil sampling in three sites where invasive and native trees were paired. In addition, in a 6-month greenhouse experiment (GHE), we grew A. altissima, R. pseudoacacia and P. alba from seeds in a soil collected from a native area. We quantified soil organic matter (OM), nitrogen (N), phosphorous (P), nitrate (NO3 eN), ammonium (NH4 þeN), pH, potential net ammonification and nitrification rates, phosphomonoesterase (PME) activity and the composition of soil bacterial community in soils from the field study and from the GHE. Both the field and the GHE results showed the capability of A. altissima to decrease soil total N and of R. pseudoacacia to increase soil mineral N. In the field, all invaded soils had greater NO3 eN than P. alba soils. R. pseudoacacia field soils had greater PME activity, total N and net ammonification rates while A. altissima soils had lower OM, NH4 þeN, net nitrification and total N mineralization rates than those of P. alba. Differences in the composition of soil bacterial communities were only found in the field, being more evident between A. altissima and P. alba than between R. pseudoacacia and P. alba field soils. Symbiotic N2 fixation could explain the capability of R. pseudoacacia to increase soil mineral N, while the potential of A. altissima to decrease total soil N may be attributed to changes in the balance between N input and losses from the soil. Although the GHE results indicated that the invasive trees can start changing soil conditions during early stages of establishment, more impacts found in the field study suggests that several soil properties, the composition of soil bacteria communities and microbial activities need longer time since invasion to be altered.